This is a randomized controlled investigator blinded study comparing single daily dosing of aminoglycoside vs divided dosing combined with beta-lactam antibiotic in children aged 6 to 18 with cystic fibrosis and acute pulmonary exacerbation. Clinical efficacy will be measured using sequential sputum bacterial titers, and pulmonary function tests. Toxicity will be evaluated using audiograms and serum creatinine. Potential cost savings will be based on length of therapy and time to subsequent hospitalization.